Process for the removal of mercury from solutions contaminated with mercury

ABSTRACT

Mercury is removed from solutions contaminated with mercury, the solution contaminated with mercury being introduced into a distillation column above the column bottom, solvent being distilled off and solution depleted in mercury being stripped off at the column bottom.

The present invention relates to a process for the removal of mercuryfrom liquids. In particular, the invention relates to a process for theremoval of mercury from solutions, especially a process for the removalof mercury from the solutions formed in the decomposition of alkalimetal amalgam by water or alcohols, especially aqueous alkali metalhydroxide solution or alcoholic alkali metal alkoxide solution.

In some chemical processes, mercury-containing liquids are obtained. Onaccount of its toxicity, mercury is usually an undesired impurity andmust therefore customarily be removed. For example, in alkali metalchloride electrolysis according to the amalgam process chlorine and analkali metal amalgam are produced. This alkali metal amalgam isdecomposed with addition of water or alcohol to obtain the productsalkali metal hydroxide or alkali metal alkoxide, which are important andproduced in large amounts, the mercury being released again and fed backinto the electrolysis. The aqueous solution of alkali metal hydroxide(customarily named “alkali solution”, especially named “sodium hydroxidesolution”, an aqueous sodium hydroxide solution and “potassium hydroxidesolution”, an aqueous potassium hydroxide solution) or alcoholicsolution of alkali metal alkoxide produced in the amalgam decompositioncontains mercury, however, always in small amounts, typically in a rangefrom 10 to 50 ppm (“parts per million”, i.e. milligrams per kilogram. Inthe context of this invention the data represent ppm or ppb, “parts perbillion”, i.e. micrograms per kilogram, always mass ratios). Undercertain circumstances, this mercury content can also achieve values ofabove 100 ppm. This amount of mercury is not tolerable for mostapplications of alkali metal hydroxides or alkali metal alkoxides andmust be lowered by a process for mercury removal (“demercurization”).Values of at most a few ppb are desired here, ideally at most 3 ppb, themercury content is thus to be lowered by a factor of approximately 10⁴.

The mercury is customarily present at least partly in the form ofmercury metal, which is usually dispersed in the liquid in the form ofvery fine droplets or—below the solubility limit—dissolved.

Various processes for the removal of mercury from product streams arealready known, also in connection with alkali metal amalgam cleavage.

EP 761 830 A2 discloses a very simple, purely mechanical process, inwhich finely divided mercury is collected in liquids by coalescence inthe form of easily separable relatively large mercury drops. Using thisprocess, a mercury depletion by a factor of at least 10 is possible, butnot down to the lower ppb range.

Often, the formation of solid amalgams is used for mercury removal. Themetals best suited for this are those of the 11th group of the PeriodicTable of the Elements, which are usually employed in the form of anabsorption mass, in which the metal is dispersed on a support. Thus DE21 02 039 discloses a process for the removal of mercury from gases suchas the hydrogen formed in the preparation of alkali metal hydroxide bycleavage of alkali metal amalgam with water, in which the gasescontaminated with mercury are brought into contact with copper on aporous aluminum oxide support and thus freed of mercury. U.S. Pat. No.4,230,486 discloses a process for the removal of mercury from liquids bypassing over a metallic silver on an absorbing agent comprising poroussupport. DE 42 21 207 A1 describes a process for the removal of mercuryfrom liquids such as alkali solutions or alkali metal alkoxide solutionsby passing over fibers coated with silver. DE 42 21 205 A1 and DE 42 21206 disclose processes for the working up of such fibers according totheir intended use. DE 41 16 890 discloses a number of absorbing agentsfor mercury removal, which contain certain metals, in particular Cu, Ag,Fe, Bi, but also Au, Sn, Zn and Pd, and mixtures of the metalsmentioned, in metallic or oxide form or as a sulfide on a supportmaterial. These processes do make possible a depletion of mercury downto the range of a few ppb, but the absorption masses used are usuallyregenerable only with very great difficulty, their absorption capacityis rapidly exhausted and they are comparatively expensive due to theconsiderable content of noble metals necessary.

The use of activated carbon having a specific surface area of at least250 m²/g for the removal of mercury from liquids by absorption is knownfrom DE 26 43 478. The use of activated carbon settling filters formercury removal from sodium hydroxide solution, potassium hydroxidesolution or alkoxide solutions is industrially customary, as describedby Isfort, Chemie Anlagen und Verfahren (“CAV”), September 1972, pages65-69. The comparatively simple activated carbon treatment, however,also does not lead to satisfactory results in all cases. In particular,to achieve the desired low mercury values a very fine-grain powder of ahigh-surface-area activated carbon must be used. Especially in thefiltration of alkoxide solutions, in which (in contrast to aqueoussolutions) very finely divided suspensions form due to side reactionswith the alcohol or cleavage of the alkoxide, the activated carbonfilters are stopped very rapidly by deposits of these suspensions, sothat only an unsatisfactory service life of the filter is achieved,which makes activated carbon treatment economically unsatisfactory. DE197 04 889 discloses as a remedy a three-stage process for thedemercurization of alkali metal alkoxide solutions, in which in a firststage the solution is led through inert fiber material in order thus toseparate off the finely divided suspensions and as a side effect tolower the mercury content by a factor of approximately 10. The solutionis filtered in a second stage through a customary activated carbonfilter having a high-surface-area and finely divided activated carbon,which lowers the mercury content in turn by a factor of approximately10. In a third stage, a distillative concentration of the alkoxide iscarried out, i.e. the solution is concentrated by evaporation of thealcohol and the desired alkoxide concentration is thus established bythe alkoxide solution being present in a distillation still and alcoholbeing distilled off through a packed column. The mercury concentrationhere again falls by a factor of approximately 10. All in all, using theprocess described mercury contents of 28 to 50 ppb are achieved. Withthis process too, the desired mercury contents of at most 3 ppb,however, cannot be achieved.

The object of the present invention is therefore to find a simple andeconomically satisfactory process with which the mercury contents inliquids, in particular in alkali solutions, but also in alkali metalalkoxide solutions, can be lowered to values of at most 3 ppb.

Accordingly, a process for the removal of mercury from solutions bydistillation has been found, which comprises introducing the solutioncontaminated with mercury into a distillation column above the columnbottom, distilling off solvent and stripping off solution depleted inmercury at the column bottom.

Surprisingly, it has now been found that by using this processconsiderably better degrees of depletion of mercury are achieved than byusing known processes or process steps. Mercury can be depleted by afactor of more than 100 using the very simple distillative processaccording to the invention. The process according to the invention is inparticular suitable for the depletion of mercury in alkali solutions andalso in alkali metal alkoxide solutions and can be combined with otherpurification processes or process steps in order to achieve higherdegrees of depletion of mercury. Repeated carrying out of thedistillative purification process according to the invention also bringsabout a correspondingly higher mercury depletion.

The solution to be purified is introduced into the column above thecolumn bottom, i.e. in plate columns at least at the height of thelowest distillation plate or in packed columns at least at the height ofthe lowest (first) theoretical plate. The fixing of the supply positionsis a simple routine task in the course of the technical engineeringdesign of the column. Preferably, the solution is introduced into thecolumn at the column top.

The solution depleted in mercury is stripped off at the column bottom.This means that the solution is removed from the column at the lower endof the column, i.e. below the actual distillation structure in which thegas/liquid exchange characterizing a distillation takes place. This canbe both the actual bottom, i.e. the lower end of the column itself,which is usually filled with liquid, and a liquid collector ordistillation receiver (“distillation still”) connected thereto.

Preferably, the process according to the invention is employed for theremoval of mercury from alkali solutions or alkali metal alkoxidesolutions, in particular those alkali solutions or alkali metal alkoxidesolutions which are produced by decomposition of alkali metal amalgamwith water or alcohol. The preparation of alkali metal amalgam and itsdecomposition with water or alcohol, uncatalyzed or using catalysts, areknown technologies. As an alkali, lithium, sodium, potassium, rubidiumor cesium is employed, preferably sodium or potassium. By decompositionof sodium or potassium amalgam with water sodium hydroxide solution orpotassium hydroxide solution is formed. By decomposition of sodium orpotassium amalgam with alcohol, a solution of the corresponding sodiumalkoxide or potassium alkoxide in the corresponding alcohol is formed.The solution or the alkoxide solution are in this case always, asdescribed above, contaminated with mercury, which is removed completelyor largely using the process according to the invention.

As an alcohol for the production of an alkali metal alkoxide solution tobe treated using the process according to the invention, any desiredalcohol can be employed. Preferably, a substituted or unsubstitutedaliphatic, alicyclic, aromatic, arylaliphatic, arylalicyclic, cycloalkylaromatic or alkyl aromatic alcohol is used. In particular, thestraight-chain or branched aliphatic alcohols having 1 to 6 carbon atomsare used, such as methanol, ethanol, 1-propanol (“n-propanol”),2-propanol (“isopropanol”), 1-butanol (“n-butanol”), 2-butanol(“isobutanol”), 2-methyl-1-propanol (“sec-butanol”),1,1-dimethyl-1-ethanol (“tert-butanol”), or the individual isomeric C5-or C6-alcohols. The use of methanol or ethanol is particularlypreferred.

By decomposition of sodium amalgam or potassium amalgam with methanol orethanol, a solution of sodium methoxide or potassium methoxide inmethanol or a solution of sodium ethoxide or potassium ethoxide inethanol is produced, which is then subjected to the process according tothe invention.

The concentration of the solution employed in the process according tothe invention—i.e., for example, the alkali solution or alkali metalalkoxide solution prepared by alkali metal amalgam decomposition withwater or alcohol, can be varied within wide ranges, it is not crucialfor the present invention. The concentration of the solutioncontaminated with mercury and the amount of solvent distilled off areadjusted such that a solution of the desired concentration and of thedesired maximum mercury content is drawn off at the column bottom.

Usually, the concentration is fixed or decisively influenced by thepreparation conditions of the solutions, in the case of the alkali metalamalgam decomposition, for example, by the amount of alcohol or wateremployed for the decomposition and by the alkali metal content of theamalgam. Therefore specific concentrations of the solution to bepurified are usually technically customary, and often subsequentprocesses are designed to these customary concentrations of theiremployed substances. The solution freed of mercury using the processaccording to the invention can then be diluted or concentrated withoutproblems and thus brought to the desired concentration. However, it isan advantage of the distillation process according to the invention thatby the choice of the concentration of the solution introduced into thecolumn and the amount of solvent distilled off a solution of the desiredconcentration can be drawn off at the column bottom. In a preferredembodiment of the process according to the invention, the amount ofsolvent which is distilled off is added to the solution before or duringthe distillation as an additional solvent and thus the original startingconcentration of the solution treated is maintained. This can be carriedout by dilution of the solution before the distillation, but also bysimultaneous introduction of solution and solvent into the distillationcolumn. In this manner, the concentration of the solution resultingduring the customary preparation of the solution and thereforetechnically also customary is maintained and only its mercury content islowered.

Provided exclusively the distillation process according to the inventionis used for the mercury separation, the concentration of the solution,for example of the alkali solution or alkoxide solution is virtuallyunimportant, a mash can also be used, that is a liquor or solution abovethe saturation concentration of the dissolved substance, i.e. having acontent of undissolved matter. In the extreme case, solid contaminatedwith mercury, for example sodium hydroxide or potassium hydroxide,sodium methoxide or potassium methoxide or sodium ethoxide or potassiumethoxide could also be introduced into the distillation column dry, butthis is technically more difficult than the introduction of a pumpablesolution or mash. The use of a pumpable solution or mash is thereforepreferred. Provided solid is introduced into the column, an amount ofadditional solvent sufficient for its dissolution is also introduced.

If, before the distillation according to the invention, furtherpurification steps are carried out, the concentration used is also to beadjusted according to the requirements of these purification steps. If,for example, filtration steps are additionally carried out, quiteobviously the use of mashes offers itself.

In the purification of sodium hydroxide solution or potassium hydroxidesolution, in general a concentration of at least 10% by weight,preferably at least 15% by weight, in a particularly preferred manner atleast 20% by weight and in general at most 70% by weight, preferably atmost 65% by weight and in a particularly preferred manner at most 60% byweight of sodium hydroxide or potassium hydroxide in water is adjusted.In the purification of sodium methoxide or potassium methoxide, ingeneral a concentration of at least 20% by weight, in a preferred mannerat least 25% by weight and in a particularly preferred manner at least27% by weight, and in general at most 40% by weight, in a preferredmanner at most 32% by weight and in a particularly preferred manner atmost 31% by weight of sodium methoxide or potassium methoxide inmethanol is adjusted. In the purification of sodium ethoxide orpotassium ethoxide, in general a concentration of at least 10% byweight, in a preferred manner at least 15% by weight and in aparticularly preferred manner at least 16% by weight, and in general atmost 30% by weight, in a preferred manner at most 22% by weight and in aparticularly preferred manner at most 20% by weight of sodium ethoxideor potassium ethoxide in ethanol is adjusted.

Provided additional solvent is added to the solution before or duringthe distillation, an additional solvent is used which has at most thesame boiling point as the solvent of the solution contaminated withmercury. In other words, the additional solvent used can be a solventthat has a lower boiling point than the solvent of the solutioncontaminated with mercury. Preferably, however, the solvent of thesolution contaminated with mercury is also used as an additionalsolvent. For the removal of mercury from liquors such as sodiumhydroxide solution or potassium hydroxide solution, water is thuspreferably used as an additional solvent, for the removal of mercuryfrom methanolic solutions of sodium methoxide or potassium methoxide andfor the removal of mercury from ethanolic solutions of sodium ethoxideor potassium ethoxide ethanol is preferably used.

The ratio of solution contaminated with mercury and additional solventand the amount of solvent distilled off are chosen such that on the onehand the desired final concentration of the purified solution isachieved and on the other hand the desired mercury depletion isachieved. Typically, a ratio of solution to additional solvent of ingeneral at least 30:1, in a preferred manner at least 20:1 and in aparticularly preferred manner at least 10:3, and in general at most 1:3,in a preferred manner 1:2 and in a particularly preferred manner at most4:1, is adjusted. The amount of solvent distilled off is then chosensuch that the desired final concentration of the purified solution isachieved.

The additional solvent is introduced either at the same position in thecolumn as the solution to be purified or in a position in the columndifferent therefrom, for example at the column bottom, at a levelbetween the bottom and column top or at the column top. Conveniently,the additional solvent is passed into the column at the same position asthe solution to be purified.

The pressure and temperature during the distillation are chosenaccording to the outline conditions present (for example according tothe heating media present at the site of the column), this is a routinetask. Typically, the pressure and temperature are chosen in atechnically customary manner as in the distillation of the solventconcerned. In the distillation of water, methanol or ethanol, normalpressure is often set and the mixture is distilled at the correspondingboiling point under normal pressure.

Mercury is separated off as a liquid phase from the solvent removed bydistillation and disposed of or preferably fed back into the preparationof amalgam again. The solvent removed by distillation is purified againor disposed of. It can also be fed back into the column again.

Provided it is the same solvent as that in the solution contaminatedwith mercury, it is preferably used for the preparation of the solution.In the preparation of alkali solution or alkali metal alkoxidesolutions, the water removed by distillation or the alcohol removed bydistillation is thus preferably fed back into the amalgam decomposer. Inthis case, a prior separation of mercury is usually unnecessary, mercurycontained in the solvent is then fed back into the mercury circulationof amalgam preparation and decomposition in the decomposer. However, itmay be advisable—as virtually always in recycling of substancestreams—to exclude and to dispose of one part stream of the recycledsolvent stream in order to prevent or to restrict an increase in thelevel of impurities (mercury and/or possible other impurities).

The embodiment of the distillation column used is not crucial for theinvention and can take place according to essentially economicconsiderations, the separation efficiency required for the removal ofthe solvent by distillation must, however, be given. The design of suchcolumns is prior art. It is possible to use, for example, plate columnsor packed columns. The use of plate columns is preferred because of thesimpler assembly of the distillation structure. Any known constructionform of column plates can be used, for example bubble-cap plates, tunnelplates or valve plates.

The mercury depletion process according to the invention can be combinedwith any other known purification process to give an overall process inorder also to combine the depletion action of the various process stepsof the overall process. For example, the combination of the processaccording to the invention with a further process using which mercury isdepleted by the factor 10 gives an overall depletion by a factor ofapproximately 10³, and the combination with two further processes usingwhich mercury can in each case be depleted by a factor of 10 gives anoverall depletion by the factor 10⁴. The sequence of carrying out theindividual process steps of the overall process can basically be chosenfreely. In general, it is advantageous first to carry out process stepswhich are suitable mainly for the removal of relatively large amounts ofmercury, in order only to carry out the final fine purification usingthe process according to the invention, without leading comparativelyhigh amounts of mercury into the distillation column. For example, it isadvisable to carry out mechanical processes such as, for example, thecoalescence of mercury droplets to give larger droplets before thedistillation process according to the invention. If, taking into accounttheir disadvantages, absorbing agents based on amalgamating noble metalsare additionally also employed, they are preferably used after thedistillation according to the invention in order thus to optimallyutilize their high purification effect with, however, low absorptioncapacity. The distillative process according to the invention makes,however, the use of such absorbing agents unnecessary, at best with theexception of some special cases with extreme purity requirements.

In a preferred embodiment of the distillation process according to theinvention for the purification, in particular, of alkali solutions oralkali metal alkoxide solutions, before or after, but preferably before,the distillation a filtration through carbon is carried out. For thisfiltration step, any of the known activated carbon filtration processesfor solutions of this type can be used. One of the advantages of theprocess according to the invention is that for a preinserted carbonfiltration step the use of a comparatively coarse carbon is sufficientand thus the blocking problems due to fine suspensions in the alkoxidesolutions is avoided. Typically, for a carbon filtration step a carbonhaving an average particle size of in general at least 10 micrometers,preferably at least 20 micrometers, and at most 1000 micrometers,preferably at most 500 micrometers, is used. The BET surface area ofsuch carbons is in general at least 0.2 m²/g, preferably at least 0.5m²/g, and in general at most 1000 m²/g, preferably at most 10 m²/g.Electrode graphite, for example, is highly suitable. A preferredelectrode graphite has a surface area of about 1 m²/g. In a customarymanner, the solution contaminated with mercury is filtered through acarbon filter which is at least 0.5 mm, preferably 1 mm, and at most 30,preferably at most 10, mm thick. For this, any filter construction canbe employed in which an appropriate carbon filter layer can bedeposited, for example flat filters, disk filters, candle filters, platefilters, suction filters, edge filters or plastic cord filter candles.The use of edge filters is preferred, onto which carbon is deposited inthe form of a suspension in the solvent used. This is well known priorart.

Furthermore, the distillation process according to the invention can becombined with a filtration step using fiber materials. These filtrationprocesses are also known. Typically, inert fibers are used, for examplefibers of polyethylene, polypropylene, polystyrene,polytetrafluoroethylene, cellulose, mineral fibers such as glass wool orrock wool, or mixtures of such fibers. The fibers are customarilypressed to give a flat structure and alternatively also sintered andprovided with a binder, filler and/or additive or a supporting fabric.These nonwoven mats contain open channels or pores and typically have aporosity in the range from 50 to 90%. They are used in customary forms,for example as disks, as filter modules, as filter candles (customarilyas cylinders having a surface area increased by pleating) or in anyother known form.

The filtration through a fiber filter can be carried out before or afterthe distillation, and before or after the filtration through carbon.Preferably, it is carried out before the distillation. Furthermore, itis carried out in a preferred manner after the carbon filtration. In avery particularly preferred embodiment of the present invention, sodiumhydroxide solution or potassium hydroxide solution, a methanolicsolution of sodium methoxide or potassium methoxide or an ethanolicsolution of sodium ethoxide or potassium ethoxide is firstly filteredthrough a carbon filter, then through a filter of inert fiber material,and then freed of mercury by passing into a distillation column,together with an additional amount of the solvent, preferably thesolution to be purified, preferably at the column top, removal ofsolvent by distillation and stripping off of the purified liquor oralkoxide solution at the column bottom.

It is also possible to carry out the filtration steps repeatedly or tocombine them in any desired manner. For example, it is possible tofilter repeatedly through carbon, repeatedly through fiber material, orrepeatedly through carbon and fiber material. The actual embodiment andsequence of individual filtration steps is a routine task of the personskilled in the art, who solves this according to the stream to betreated, its impurity content and the requirements of the depletion.

Using the process described, a very simple depletion of mercury to thevalues achievable by use of amalgamating noble metals is possible, i.e.to at most 3 ppb, without the disadvantages of the amalgamating noblemetals having to be expected.

EXAMPLES Example 1

An edge filter was coated with a 2 to 3 mm thick carbon layer bydepositing a suspension of electrode graphite (mean particle size 300micrometers, BET surface area 1.1 m²/g) in methanol. At a temperature of70-80° C., a mercury-polluted methanolic sodium methoxide solution (27%by weight) was filtered at a flow rate of 12-15 liters per 100 cm2filter area per hour. The solution running off was then added to the topof a continuously operated tunnel plate column together with 20 litersof methanol per 100 liters of sodium methoxide solution. Sufficientmethanol was removed by distillation such that at the column bottom a30% strength by weight sodium methoxide solution was obtained.

Before and after filtration, and after the distillation, 4 samples ineach case were taken and analyzed for their mercury content. The resultsare shown in the following table.

Before filtration After filtration After distillation

Before filtration After filtration After distillation [ppm] [ppm] [ppb]23 3.3 20 21 4.3 10 18 3.3 14 17 3.1 12

The values show that using the distillation process according to theinvention degrees of mercury depletion of markedly more than 100 areachieved and thus despite use of a relatively coarse filter carbon andusing an only two-stage process even lower mercury contents are achievedthan with the process of DE 197 04 889, in which the solutioncontaminated with mercury is introduced into a distillation still andonly the solvent is stripped off.

Example 2

A carbon filtration was carried out with a 27% by weight sodiummethoxide solution as in example 1. The methoxide solution was thendiluted with methanol in the ratio 5:1 and filtered through a diskfilter module having a filter layer of a mixture of cellulose fiberswith kieselguhr having pore widths in the range of 2 to 5 micrometers.The solution was then added to the top of a continuously operated tunnelplate column without further dilution. Sufficient methanol was distilledoff such that a 30% strength by weight sodium methoxide solution wasobtained at the column bottom.

Before and after filtration, and after the distillation, 5 samples ineach case were taken and analyzed for their mercury content. The resultsare shown in the following table.

Before filtration After carbon After filtration through fiber [ppm]filtration [ppm] material and distillation [ppb] 18 2.3 1-2 19 3.5 2 192.9 1 19 2.3 2 >100 1.8 <1

Example 3

A carbon filtration was carried out using a 27% strength by weightsodium methoxide solution as in example 1. The methoxide solution wasthen diluted with methanol in the ratio 5:1.1 and filtered through acandle filter module using filter candles of polypropylene fibernonwoven. The candles were loaded at 460 to 560 l/h, based on a filterelement having a 10 inch length. The solution was then added to the topof a continuously operated tunnel plate column without further dilution.Sufficient methanol was distilled off such that a 30% strength by weightsodium methoxide solution was obtained at the column bottom.

Before and after filtration, and after distillation, 2 samples in eachcase were removed and analyzed for their mercury content. The resultsare shown in the following table.

Before filtration After carbon After filtration through fiber [ppm]filtration [ppm] material and distillation [ppb] 10.8 3.6 1 13.8 4.2 1

Examples 1 and 2 show that using the distillation process according tothe invention mercury can be depleted down to the lowest ppb range withan outlay comparable to previously known processes.

1. A process for the removal of mercury from an alkali solution or analcoholic alkali metal alkoxide solution contaminated with mercury,wherein the alkali solution or alcoholic alkali metal alkoxide solutioncontaminated with mercury is first filtered through carbon, thenfiltered through an inert fiber material and then the alkali solution orthe alcoholic alkali metal alkoxide solution is distilled wherein thedistillation comprises introducing the solution into a distillationcolumn above the column bottom, distilling off water or alcohol andremoving the alkali solution or alcoholic alkali metal alkoxide solutiondepleted in mercury at the column bottom.
 2. A process as claimed inclaim 1, wherein apart from the alkali solution or alcoholic alkalimetal alkoxide solution contaminated with mercury, additional solvent isintroduced into the column.
 3. A process as claimed in claim 2, whereinthe alkali solution or alcoholic alkali metal alkoxide solutioncontaminated with mercury and additional solvent is introduced into thecolumn in a volume ratio in the range from 30:1 to 1:3.
 4. A process asclaimed in claim 1, wherein the alkali solution or alcoholic alkalimetal alkoxide solution contaminated with mercury is introduced at thecolumn top.